The invention broadly relates to a Raster Output Scanner (ROS) imaging system, and, more particularly, to a means and method for generating timing signals responsive to the detection of a scanning beam crossing a fiber optic detector.
In conventional ROS systems, an intensity modulated light beam generated by a gas or diode laser is repetitively scanned across the surface of a photosensitive image plane to form a latent image of a document or the like represented by input binary data. Each scan line comprises composite images of individual pixels representing on and off states of the laser. These pixels must be aligned from scan to scan in the vertical or fast-scan direction; failure to do so results in the phenomenon known as scan line "jitter". It is known in the prior art that photodetectors can be positioned in the scan path a predetermined distance upstream from the recording surface where their output is used to generate a Start Of Scan (SOS) signal controlling the timing of the laser modulation waveform. Exemplary of the known detectors is a slit detector design in which the amplitude of the photodiode output signal is compared with a predetermined fixed reference voltage. When the scanned laser beam passes over the photodetector surface, the amplitude of the output signal reaches this reference threshold and an SOS pulse is generated. Also known is the so-called split detector which utilizes two adjacent photodiodes in very close proximity in an electronic comparator configuration that compensates for variations in scanning beam power. In operation, the sweep of the beam over the first detector establishes a dynamic reference level for the second detector that is proportional to the intensity of the scanned light beam. With this arrangement, the comparator is triggered when the swept beam is positioned at the midpoint between the detectors and the light levels in both detectors match exactly. An example of a split detector is disclosed in U.S. Pat. No. 4,386,272.
For many high speed, high resolution Raster Output Scanner (ROS) systems, a solid state laser diode or a HeNe laser is the preferred device for generating the recording beams. As is well known, the power output of these lasers varies in amplitude over time. The conventional slit detector, when used with a laser scanning system, is subject to jitter because the output current of the photodetector responds proportionately, in amplitude, to the Gaussian shape of the scanned beam as it sweeps across the face of the detector. Outputs produced by beams of different power levels will, necessarily, reach the fixed reference level at different relative times, resulting in SOS outputs at different times relative to passage of the center of a scanned Gaussian beam. Since the synchronization of the electronic system that controls the timing of the information bit stream defining the laser modulation waveforms for each line is keyed to the SOS pulse, this differential triggering effects a net translation of the exposure pattern of each scan line in the fast scan direction. As a result, the alignment of picture elements in the exposure raster from line to line is inexact.
The split detector generates an SOS output when the Gaussian beam is centered between the two photodetector sites. Since the response depends only on the relative position of the beam and not on a specific amplitude level, the SOS output signal timing is independent of the beam power. In other words, the split detector generates an SOS signal which does not vary in time when the diode intensity changes. Both the slit detector and the split detector are typically configured in the same fashion; the photodetector elements and associated amplifiers and pulse shaping electronics are assembled in a remote housing which is positioned adjacent to the imaging surface in or very close to the focal plane path of the scanned beam. SOS pulses from the detector assembly are returned via coaxial cable or twisted pair to a central electronic network containing the image data, system timing, and laser modulation circuitry.
A third detection method is known in the art wherein the position of a scanning laser beam is sensed by placing an optical fiber or light pipe in the path of the scanning beam to transmit the incident light to the central electronics system. The conveyed light energy is incident on an indicia as, for example, disclosed in U.S. Pat. No. 4,071,754, or on a photodetector, located on a central circuit board of the electronics system. The detector converts the light energy into an electrical signal which is then processed to provide synchronization signals for the laser. Another application discloses a fiber optic detector which uses a single optical fiber positioned in the scan path at the beginning of a scan line sweep. The fiber transmits a portion of the scan beam flux to a photodetector located on a central electronics circuit board. The photodiode generates an output signal which drives one input of a high speed comparator. The second comparator input is fed an amplified and delayed analog of the photodetector output signal. The comparator senses the difference in the two voltage wave forms and generates an output transition at the precise time the two wave forms cross over or intersect. The comparator output transition is used to initiate the scanning system SOS signal of a gas or laser diode ROS. Fiber optic detectors have several advantages over the split and slit detectors; they are more compact, less expensive, provide superior noise immunity and have simple mechanical mounting. Lower cost is realized because separate scan detector circuit boards and housings are not needed and because the cable and connectors that provide power and signals to and from the remote scan detector board are unnecessary. Noise immunity is superior because the laser printer environment is electrically noisy (EMI, RFI) and the remote scan detector and its cables are difficult to shield from this noisy environment. The optical fiber simply acts as a light flux conduit through the noisy environment to the local electronics board where the light signal is converted to an electronic signal in a controlled environment (shielded) where signal traces are short and noise is easier to control. Further, the mechanical mounting of the remote detector system is often awkward because the scanner footprint is typically narrow near the ROS image plane and the space available for the scan detector electronics board is relatively cramped. The optical fiber is very small and so is easier to locate and mount.
In the prior art fiber optic detection systems, the detected light, typically from a ROS scanning beam, is coupled axially into the input end of the fiber or a group of fibers. FIG. 1 shows a single large multimode optical fiber 2 with circular input and output end faces 4 and 6, respectively. A scanning beam 5 sweeps across the input end 4 and flux transmitted along the fiber axis is emitted at the output end 6. If a photosensor element is placed in close proximity to the light output end 6, flux from passage of the beam at the input end 4 will generate a photocurrent pulse waveform profile 10 representing the detected pulse. The transitions of this waveform may be too slow and the overall pulse length may be too long for the typical high speed circuitry used to generate the start of scan signals required for ROS images. FIG. 2 shows an optical fiber whose input end 4' has been changed to a rectangular configuration presenting a shorter face to the scanning beam. End 6' retains a circular configuration. End 4' can be deformed, for example, by the techniques disclosed in U.S. Pat. No. 4,952,022, whose contents are hereby incorporated by reference. Profile 10' is shown as relatively shorter than profile 10. End 4' can also be reshaped into a widened cross section so that subsequent alignment of the fiber end with respect to the scan path is less critical. These prior art configurations require that the input end of the fiber detector be oriented so that the scanning light beam flux enters the fiber axially, within a limiting cone angle determined by the numerical aperture (NA) of the fiber, in order to propagate, along the fiber length. It would be advantageous to remove this design constraint so that the fiber could assume other, non-axial, orientations relative to the scanning beam.
It is therefore an object of the invention to provide a fiber optic detection system wherein the detected light enters the fiber from a non-axial direction via an input face.
It is a further object to provide embodiments combining two or more optical fibers, each of the fibers receiving light non-axially.
These and other objects are realized by forming light admitting windows or facets in areas on or near the fiber entrance end using combinations of reflecting surfaces and the principle of total internal reflection to direct the light, once introduced non-axially into the fiber, axially down the fiber to a photosensor optically coupled to the exit end. In one embodiment, the fiber end is shaped so that light enters the fiber through a flattened window portion along the longitudinal surface. Once within the fiber, light flux strikes an angled end face and is reflected axially along the fiber length. In another embodiment, multiple fibers with their ends deformed into rectangles with an angled facet are stacked in an array, and a scanning beam is introduced into the fibers through windows formed along their edges.
More particularly, the invention relates to a fiber optic scanning beam detector comprising:
fiber optic means positioned in the path of a scanning beam of light to intercept said beam, said fiber optic means transmitting light energy from said intercepted beam onto a photosensor thereby causing said photosensor to generate an electrical signal corresponding to the intensity of said intercepted light energy, said fiber optic means comprising at least one optical fiber with at least a light entrance end and a light exit end with the entrance end modified so that the intercepted light enters the entrance end of the fiber in an non-axial direction.